The eROSITA Final Equatorial Depth Survey (eFEDS)

The hard X-ray selected sample

¹ Max Planck Institute for Extraterrestrial Physics (MPE), Giessenbachstrasse 1, 85748 Garching bei München, Germany
² Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
³ Dipartimento di Fisica e Astronomia ‘Augusto Righi’, Alma Mater Studiorum Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁴ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁵ Institute for Astronomy and Astrophysics, National Observatory of Athens, V. Paulou and I. Metaxa, 11532, Greece
⁶ Dr. Karl Remeis-Sternwarte and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁸ Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama, Copayapu 485, Copiapó, Chile
⁹ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁰ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago 7820436, Chile
¹¹ Centro de Astroingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Campus San Joaquín, Av. Vicuña Mackenna 4860, Macul Santiago 7820436, Chile
¹² Millennium Institute of Astrophysics, Nuncio Monseñor Sótero Sanz 100, Of 104, Providencia, Santiago, Chile
¹³ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
¹⁴ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
¹⁵ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹⁶ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
¹⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

^★ Corresponding author; knandra@mpe.mpg.de

Received: 30 January 2024
Accepted: 12 July 2024

Abstract

Context. During its calibration and performance verification phase, the eROSITA instrument aboard the Spectrum-RG satellite performed a uniform wide-area X-ray survey of approximately 140 deg², known as the eROSITA Final Equatorial Depth Survey (eFEDS).

Aims. The primary aim of eFEDS is to demonstrate the scientific performance to be expected at the end of the eight–pass eROSITA all-sky survey. This survey will provide the first focussed image of the whole sky in the hard X-ray (>2 keV) bandpass. The expected source population in this energy range is thus of great interest, particularly for AGN studies.

Methods. We used a 2.3–5 keV selection to construct a sample of 246 point-like hard X-ray sources for further study and characterisation. These sources are classified as either extragalactic (∼90%) or Galactic (∼10%), with the former consisting overwhelmingly of AGN and the latter active stars. We concentrated our further analysis on the extragalactic AGN sample, describing their X-ray and multi-wavelength properties and comparing them to the eFEDS main AGN sample selected in the softer 0.2–2.3 keV band.

Results. The eROSITA hard band selects a subsample of sources that is a factor of more than ten brighter than the eFEDS main sample. The AGN within the hard population reach up to z = 3.2 but on the whole, they are relatively nearby, with median z = 0.34 compared to z = 0.94 for the main sample. The hard survey probes typical luminosities in the range log L_X = 43–46. The X-ray spectral analysis shows significant intrinsic absorption (with log N_H > 21) in ~20% of the sources, with a hard X-ray power law continuum with mean < Γ >= 1.83 ± 0.04, which is typical of AGN, but slightly harder than the soft-selected eROSITA sample. Around 10% of the hard sample show a significant ‘soft excess’ component. The sampled black hole mass distribution in the eFEDS broad-line AGN population is consistent with that of the deeper COSMOS survey that probes a higher redshift population. On the other hand, the Eddington ratios appear systematically lower, which is consistent with the idea that the decline in SMBH activity since z ∼ 1 is due to a reduction in the typical accretion rate, rather than a shift towards activity in lower-mass black holes.

Conclusions. The eFEDS hard sample provides a preview of what can be expected from the eRASS final survey in terms of data quality. This pilot survey indicates the power of eROSITA to shed new light on the demographics and evolution of AGN, and the potential for discovery of new and rare populations.